Compensated transformer circuit utilizing negative capacitance simulating circuit

ABSTRACT

A circuit for reducing or eliminating the effect of the stray capacitance of the windings of a transformer on signals coupled through that transformer. Circuitry is provided for generating voltages and currents which simulate the presence of a negative capacitance and for coupling those voltages and currents to a transformer in cancelling relationships to the stray capacitance thereof. Circuitry is also provided for imposing an upper frequency limit beyond which capacitance cancellation will not occur. The upper frequency limit stabilizes the circuitry and allows signal transmission to be limited to a predetermined desired band of frequencies.

United States Patent [191 Kiko [11 3,832,654 Aug. 27, 1974 COMPENSATEDTRANSFORMER CIRCUIT UTILIZING NEGATIVE CAPACITANCE SIMULATING CIRCUIT[75] Inventor: Frederick J. Kiko, Sheffield Village,

Ohio

[73] Assignee: Lorain Products Corporation,

Lorain, Ohio [22] Filed: Dec. 20, 1973 21 Appl. N6; 426,826

[52] US. Cl. 333/24 R, 323/44 R, 323/48, 333/80 R [51] Int. Cl. H03h7/00, I-IO3h 11/00 [58] Field of Search..... 336/69; 333/17, 80 R, 80 T,333/12, 24 R, 24 C; 323/44 R, 48

[56] References Cited UNlTED STATES PATENTS 3.013.225 12/1961 Ouchi3,474,355 10/1969 Schlichte 333/80R 3,546,565 12/1970 Downing, Jr. eta1. 323/48 X 3,684,948 8/1972 Eissmann 323/48 X Primary Examin'erPaul L.Gensler Attorney, Agent, or Firm-Edward C. Jason [57] ABSTRACT A circuitfor reducing or eliminating the effect of the stray capacitance of thewindings of a transformer on signals coupled through that transformer.Circuitry is provided for generating voltages and currents whichsimulate the presence of a negative capacitance and for coupling thosevoltages and currents to a transformer in cancelling relationships tothe stray capacitance thereof. Circuitry is also provided for imposingan upper frequency limit beyond which capacitance cancellation will notoccur. The upper frequency limit stabilizes the circuitry and allowssignal transmission to be limited to a predetermined desired band offrequencies.

18 Clains, 15 Drawing Figures PAIENFED saw 1 or 1:.

PAENIEU we 2 71974 sun-2m 4 FIG. 2b.

PATENIED MIG 2 7 I974 SIEUJNd :lines which include such transformers.

'COMPENSATEDIRANSFQRMER a v t UTILIZING NEGATIVE CAPACITANCE SIMULATINGV3 m .15 v.

- I BACKGROUND oFTHEINv NnoNT The present invention relates tocompensating circuits for transformers'and is' directed-moreparticularlyto circuitsfor reducing the effect of stray capacitance on signaltransmission through transformers. I

Becasue of their usefulness in providingisolation, impedance matchingand voltage and'currenttransformations, transformers have become widelyused in circuits ,such as voice-frequency amplifiers. In avoicefrequency amplifier circuit such as, for example,;th'at describedin the US. Pat. of Charles W. Chambers, Jr.,

No. 3,706,862, entitled Amplifier-Circuit For Trans- 1 mission Lines,transformers play a useful 'role in coutelephone line. Q

One serious problem pling the amplifiercircuitry-in series. withandacrossa With utilizing transformers for' coupling voice-frequencysignals is that, the windings thereof exhibit a stray, distributedcapacitance between the'turns of the windings thereof. This straycapacitance causes a portion of the driving signal current'to traversethe terminals of the primary winding without inducing correspondingcurrents in the secondaryembodiment of the I the invention,

cuitry for setting I =Yet another-object of the invention is to providecircuitry which substantially cancels the effect of the stray,distributed capacitance of the'windings of transforrners. v i

Still another object of the invention is to provide circuitry, of theabove character includingcircuitry for suppressing capacitancecancellation at frequencies beyond the'ban'd in which capacitancecancellation is desirable. i

' Another objectof the invention is to provide cir- 'cuitry of the abovecharacter which can be used in'existing transformers. I

I a DESCRIPTIONOE THEDRAWINGS I FIGS. 1, la andglbareschematicdiagramsof one il lustrative embodiment of the compensated transformercircuit of the invention,

FIGS; 2, 2a. and 2b. are-schematic diagrams showing. a second embodimentof the compensated transformer circuit-of the invention,

FIGS. 3, 3a'and 3b are schematic FIGS. 4, s and 6 are climatic-diagramsa further embodiments of compensated transformer circuits of theinvention, and I FIGS, 7, 8 and 9 are 'schernaticf diagrams of embodiments of the invention which include band-limiting ciran upper frequencylimit beyond which compensation does not occur..

DESCRIPTION 05' THEINVENTION Q I Referringto FIG. rmeris'j shown asource of a-c nerwas, however, quite limited and not subject to changeonce the transformer was made. ln addition, such improvements could notbe made in existing transformers." i a a n In accordance with thepresent, invention, .there'is provided capacitance compensatingcircuitry which improves the high frequency response of transformershaving'ordinary core materials and ordinary'winding configurations, Morespecifically, the compensating circuitry of the invention isadaptedtosubstantially cancel the stray capacitance of transformers and therebyextend the band of frequencies which may be coupled through thetransfonner without significant frequency dependentetfects'ln addition,the circuit of the invention isv adapted to allow the desiredcapacitance cancellationto occur over a controllable band of frequenciesextending far beyond that normally provided by ordinary transformers andto suppress that capacitance 'cancelllationfor signal frequenciesoutside ofthedes ed band. Y I V i i SUMMARY OF THE INVENTION v It is anobject of the invention to provide circuitry for i is desirable; toelectrically isolate the repeater circuit voltage 10 for energizing aload 12- through a transformer 14. Source 10 may, for-example, comprisea transmission line which is energized by a remote voicefrequency sourceand load12 may comprise a repeater circuit which amplifies thetransmission of signals through; that transmissionline.-ln transmissionline-' repeater systems of this typ a transformer such as 14 fromithe.transmission line and vice-versa'.v I l In the present embodiment,transformer 14 .comprises asuitablemagn'eticcore 14a around whichare.disposed a primary winding 14b, a secondary-winding l4c-and a tertiaryor auxiliary winding 14d. Because of. the adjacency of the turns ofwindings 14b and ,14c, transformer 14 exhibits a stray capacitance,which may be visualized as a first lumped capacitance connected betweenprimary winding terminals P1 and P2 and a second lumped capacitanceconnected between secondary winding terminals S1 and S2. Alternatively,the

lumped, stray capacitance may be visualized entirely across primarywinding terminals P1 and P2 or entirely across secondary windingterminals S1 and S2. Theeffect of this stray capacitance isto'shuntsignal current across the turns of the primary and secondarywindings andthereby reduce the percentage of the signal from improvingthe signal distortion characteristicsof ordi nary transformers.

'Another object of the invention is to provide compensating circuitryfor substantially reducing or eliminating the effect of frequencydependent current flow,

through the stray capacitance of transfonner windings.-

components of thesignal from affecting load is ordinarily so'low in thelow the effect thereof may source 10 that is'transformed in'transformer14. The magnitude of the current through thestray capacitance and middlefrequency regions of,-for;example,-;the voice-frequency band that beneglected..At the high end of the voice-frequency band, however, thecurrent through the stray. capacitance is substantialvents'anappreciable portion of the high fr 12; This diagramsofa'thirdcompensated transformer circuit of and preequency.

thereof, voltages and currents which effectively cancel the straycapacitance of transformer 14. As a result,

simulating network 16 allows signal transmission through transformer 14to proceed as if that transformer had no stray capacitance and therebygreatly reduces the distortion of the high frequency components of thesignal.

' In accordance with one feature of the present invention, the desiredcapacitance cancellation is produced by simulating between terminals 16aand 16b the presence of a negative capacitance having a magnitude which,through winding 14d, approximately equals the real stray capacitance oftransformer 14. The result of applying this simulated negativecapacitance to transformer 14 is that the negative or simulatedcapacitance combines in parallel with the positive or real straycapacitance, according to the usual parallel combination rules, togenerate an equivalent combined capacitance having a value approximatelyequal to zero. The latter condition effectively prevents transformer 14from affecting the high frequency components of signal transmission fromsource to load 12.

In the present embodiment, simulating network 16 includes an operationalamplifier 18 having an inverting input 18a, a noninverting input 18b,and an output 18c. Simulating network 16 also includes feedback meanscomprising a negative feedback impedance here shown as an inductor 20connected between amplifier output 180 and inverting amplifier input18a, a positive feedback impedance here shown as a resistor 22 connectedbetween amplifier output 180 and non-inverting amplifier input 18b, andan input feedback impedance here shown as a resistor 24 connectedbetween input terminal 16a and inverting amplifier input 18a. Therelationship governing the magnitude of the impedance Z looking intoterminals 16a and 16b of this type of circuit, that is, a circuitwherein the input feedback impedance is connected to the invertingamplifier input is given by the formula Z, Z Z /Z where Z, is thenegative feedback impedance, Z is the input feedback impedance and Z isthe positive feedback impedance. This relationship allows the values ofinductor 20 and resistors 22 and 24 to be selected to afford thenecessary value. of negative capacitance.

In the event that it is desirable to provide stray capacitancecancellation in transformers which do not have a tertiary winding as,for example, where it is desirable to add the compensating circuit ofthe invention to an existing two-winding transformer, this may beaccomplished by connecting simulating network 16 to the existingtransformer in the manner shown in FIG. la or lb. The circuits of FIGS.la and lb are generally similar to that of FIG. 1 and like functioningparts are similarly numbered. It will be understood, however, that thecomponent values utilized in the circuits of FIGS. la and lb are notnecessarily the same as those utilized in the circuit of FIG. 1.

As shown in FIG. 1a, the input terminals of simulating network 16 may beconnected across primary winding 14b of transfonner 14. In thisposition, simulating network 16 operates in the manner described inconnection with the circuit of FIG. 1 to substantially cancel the straycapacitance of transformer 14. Similarly, as shown in FIG. 1b,capacitance cancellation may be provided by connecting the inputterminals of simulating network 16 across the secondary winding oftransformer 14. Thus, the circuit of the invention may be applied toexisting two-winding transformers as well as to specially woundthree-winding transformers.

The desired negative capacitance may also be real-' ized with thefeedback circuitry shown in FIG. 4. The circuit of FIG. 4 is similar tothat of FIG. 1, like functioning parts being similarly numbered. Thecircuit of FIG. 4 differs from that of FIG. 1 primarily in that in FIG.4 inductor 20' is a positive feedback element connected to non-invertingamplifier input 18b, resistor 22' is a negative feedback elementconnected to inverting amplifier input 18a and resistor 24' is an inputfeedback element connected to non-inverting amplifier input 18b. Incircuits of the type shown in FIG. 4, that is, circuits wherein theinput feedback impedance is connected to the non-inverting amplifierinput, the relationship governing the magnitude of the impedance Zlooking into terminals 16a and 16b is given by the formula Z, =Z 21/23,where Z, is the negative feedback impedance, Z; is the input feedbackimpedance and Z is the positive feedback impedance. Thus, capacitancesimulating circuit 16 may be realized with an inductive reactance ineither the negative or the positive feedback path. It will be understoodthat capacitance simulating circuit 16 of FIG. 4 may be coupled totransformer 14 by metallic connections across the primary or secondarywinding, as shown in FIGS. la and 1b.

The circuit of FIG. 2 shows an embodiment of the invention in whichcapacitance cancellation is afforded without utilizing an inductor inthe feedback path of amplifier 18. The circuit of FIG. 2 is generallysimilar to that of FIG. 1 and like functioning parts are similarlynumbered. The circuit of FIG. 2 differs from that of FIG. 1 primarily inthat in the circuit of FIG. 2 the simulation of negative capacitancebetween terminals 16a and 16b is accomplished by means of a capacitivereactance 28 connected in the input feedback path of amplifier 18,rather than by an inductive reactance in the negative feedback path ofamplifier 18, as shown in FIG. 1. The effect produced by the circuit ofFIG. 2 and the manner in which it is produced is, however, the same asthat described in connection with the circuit of FIG. 1.

FIGS. 2a and 2b show embodiments of the invention wherein the simulatingnetwork of FIG. 2 is connected, respectively, across the primary andsecondary windings of transformer 14. As in the case of FIGS. la and 1b,the primary and secondary winding connections shown in FIGS. 2a and 2ballow simulating network 16 to be applied to existing two-windingtransformers. Thus, the circuits of FIGS. 2a and 2b bear the samerelationship to the circuit of FIG. 2 that the circuits of FIGS. la andlb bear to the circuit of FIG. 1.

Referring to FIG. 5, there is shown another embodiment of the inventionin which a capacitive input feedback impedance is utilized to simulatethe desired negative capacitance between terminals 16a and 16b. The

circuit of FIG. 5 is similar to that of FIG. 2 and like functioningparts are similarly numbered. The circuit of FIG. 5 differs from that ofFIG. 2 primarily in that the input feedback capacitor 28 of FIG. 4 isconnected to non-inverting amplifier input 18b rather than to invertingamplifier input 18a, as shown in FIG. 2. Thus, the circuit of FIG. 5will be seen to bear the same relationship to the circuit of FIG. 2 thatthe circuit of FIG. 4 did to the circuit of FIG. 1. It will beunderstood that a circuit of the type shown in FIG. 5 may also becoupled to transformer 14 by metallic connections across the primary orsecondary winding as shown in FIGS. 2a and 2b.

The circuit of FIG. 3 shows an embodiment of the invention in whichcapacitance cancellation is afforded by a simulating network having acapacitor 30 in the positive feedback path of amplifier 18 and resistors26 and 32 in the respective negative and input feedback paths thereof.The effect produced by simulating network l6 of FIG. 3 and the manner inwhich that effect is produced is, however, the same as that described inconnection with the circuit of FIG. 1.

FIGS. 3a and 3b, show embodiments of the invention viously describedembodiments of the invention. Referring to FIG. 7, for example, there isshown a compensated transformer circuit of the type shown in FIG. 1which includes a band-limiting network here shown as a capacitor 34 inparallel with a resistor 36. At the low and middle frequency regions ofthe desired band, capacitor 34 and resistor 36 have only a negligibleeffect and allow inductor 20 to cooperate with resistors 22 and 24 toproduce the desired negative capacitance simulation. At frequenciesbeyond the high end of the desired band, however, the effect ofcapacitor 34 increases with frequency to cancel an increasing proportionof the capacitance simulating effect of inductor 20. This occurs becausethe reactance of capacitor 34 decreases with frequency and therebycauses overall impedance of the negative feedback path to decrease withfrequency, even though the reactance of inductor 20 wherein thesimulating network of FIG. 3 is connected,

respectively, across the primary and secondary windings of transformer14. As in the cases of FIGS. 1a and lb, and 2a and 2b, the primary andsecondary winding connections shown in FIGS. 3a and 3b allow simulatingnetwork 16 to be applied to existing two-winding transformers. Thus, thecircuits of FIGS. and 3b bear the same relationship to the circuit ofFIG. 3 that the circuits of FIGS. la and lb, and 2a and 2b bear to thecircuits of FIGS. 1 and 2, respectively.

Referring to FIG. 6, there is shown another embodiment of the inventionin which a capacitive feedback impedance is utilized to simulate thedesired negative capacitance between terminals 16a and 16b. The circuitof FIG. 6 is similar to that of FIG. 3 and like functioning parts aresimilarly numbered. The circuit of FIG. 6 differs from that of FIG. 3primarily in that capacitor 30 of FIG. 6 is connected in the negativefeedback path of amplifier 18 rather than in the positive feedback pathof amplifier 18, as shown in FIG. 3. Thus, the circuit of FIG. 6 will beseen to bear the same relationship to the circuit of FIG. 3 that thecircuits of FIGS. 4 and 5, respectively, bore to the circuits of FIGS. 1and 2. It will be understood that a circuit of the type shown in FIG. 6may also be coupled to transformer 14 by metallic connections across theprimary or secondary winding as shown in FIGS. 30 and 3b.

In view of the foregoing, it will be seen that the compensatedtransformer circuit of the invention may be realized with eitherinductive or capacitive feedback elements and that such inductive orcapacitive elements may be connected in the negative feedback path, thepositive feedback path or the input feedback path. It will further beseen that each of the embodiments of the invention may be realized byconnecting the capacitance simulating network to the transformer througha tertiary winding, by a metallic connection across the primary windingor by a metallic connection across the secondary winding.

To the end that the capacitance simulating activity of network 16 may besuppressed at frequencies above the bandof frequencies within whichcapacitance cancellation is desired, there is provided hereinbandlimiting circuitry which is applicable to each of the preincreaseswith frequency for frequencies beyond the desired transmission band.Thus, capacitor 34 reduces the magnitude of the simulated negativecapacitance, as a function of frequency, for frequencies beyond thedesired transmission band.

In order to stabilize capacitance simulating network 16 in the presenceof the parallel resonance condition which arises between inductor 20 andcapacitor 34, a resistor 36 may be connected in parallel with inductor20 and capacitor 34. The effect of this resistor is to place an upperlimit on the magnitude of the impedance of the negative feedback pathand thereby limit the gain of amplifier 18. In other words, the resistor36 may be said to spoil the Q of the tank circuit including inductor 20and capacitor 34.

Referring to FIG. 8 there is shown a compensated transformer circuit ofthe type shown in FIG. 2 which includes a band-limiting network hereshown as an inductor 38 and resistor 40. The latter elements areconnected in series with input feedback capacitor 28 to cancel thecapacitance simulating effect of capacitor 28 for frequencies beyond theupper end of the desired transmission band. This occurs because thereactance of inductor 38 increases with frequency and thereby causes theoverall impedance of the input feedback path to increase with frequency,even though the reactance of capacitor 28 decreases with frequency forfrequencies beyond the desired transmission band. Thus, inductor 38decreases the magnitude of the simulated negative capacitance, as afunction of frequency, for frequencies beyond the desired transmissionband.

In order to stabilize capacitance simulating network 16 in the presenceof a series resonant condition in capacitor 28 and inductor 38, asresistor 40 may be connected in series in the input impedance path. Theeffect I of this resistor is to place a lower limit on the magnitude ofthe impedance of that branch and thereby limit the gain of amplifier 18.In other words, resistor 40 may be said to -spoil the Q of the seriesresonant circuit including capacitor 28 and inductor 38.

Referring to FIG. 9, there is shown a compensated transformer circuit ofthe type shown in FIG. 3 which includes band-limiting circuitry hereshown as an inductor 42 and a resistor 44. The latter elements areconnected in series with capacitor 30 in the positive feedback path ofamplifier 18. In this location, inductor 42 serves to cancel thecapacitance simulating effect of capacitor 30 for frequencies beyond theupper end of the desired transmission band in a manner generally similarto that described in connection with FIGS. 7

and 8. As in the case of resistors 36 and 40 in FIG. 7 and 8, resistor44 is provided to stabilize the response of the capacitance simulatingnetwork at frequencies near those at which capacitor 30 and inductor 42approach a condition of resonance.

In view of the foregoing, it will be seen that the bandlimitingcircuitry of the invention may be applied in embodiments of theinvention wherein a reactance is connected in the negative feedbackpath, in the positive feedback path or in the input feedback path. Itwill further be seen that the applicability of the bandlimitingcircuitry of the invention is unaffected by the manner in which thecapacitance simulating circuitry is coupled to transformer 14, that is,whether the coupling is by means of a tertiary winding such as 14d or bymetallic connections across the primary or secondary windings.

It will be understood that the embodiments shown herein are for,illustrative purposes only and may be changed or modified withoutdeparting from the spirit and scope of the appended claims.

What is claimed is:

l. A compensated transformer circuit comprising, in combination, atransformer including a primary and a secondary winding, capacitancesimulating means for establishing voltages and currents which affect thecircuitry to which said voltages and currents are applied as if anegative capacitance were connected to said circuitry, said negativecapacitance having a magnitude substantially proportional to the straycapacitance of said transformer, and coupling means for coupling saidcapacitance simulating means to said transformer to substantially cancelthe effect which said stray capacitance has on the voltages across andcurrents through said primary and secondary windings.

2. A compensated transformer circuit as set forth in claim 1 in whichsaid coupling means comprises a tertiary winding in said transformer.

3. A compensated transformer circuit as set forth in claim 1 in whichsaid coupling means includes means for connecting said capacitancesimulating means across said primary winding.

4. A compensated transformer circuit as set forth in claim 1 in whichsaid coupling means includes means for connecting said capacitancesimulating means across said secondary winding.

5. A compensated transformer circuit as set forth in claim 1 includingband-limiting means for placing an upper frequency limit on the band offrequencies within which said capacitance simulating means cancels saidstray capacitance.

6. A compensated transformer circuit including, in combination, atransformer having a primary and a secondary winding, a capacitancesimulating circuit comprising a plurality of terminals, an amplifier, aplurality of feedback elements and means for connecting said feedbackelements to said amplifier to establish between said terminals aneffective negative capacitance having a magnitude substantiallyproportional to the stray capacitance of said transformer, saidcompensated transformer circuit further including coupling means forcoupling said terminals to said transformer to substantially cancel theeffect which said stray capacitance has on the transformation of voltageand cur rent in said transformerf 7. A compensated transformer circuitas set forth in claim 6 including band-limiting means for placing anupper frequency limit on the band of frequencies within which saidcapacitance simulating circuit cancels said stray capacitance.

8. A compensated transformer circuit as set forth in claim 6 in whichsaid amplifier is an operational amplitier of the type which includes aninverting input, a non-inverting input and an output and in which saidplurality of feedback elements includes a positive feedback impedance, anegative feedback impedance and an input feedback impedance.

9. A compensated transformer circuit as set forth in claim 8 whereinsaid input feedback impedance is connected to said inverting input.

10. A compensated transformer circuit as set forth in claim 8 whereinsaid input feedback impedance is connected to said noninverting input.

1 1. A compensated transformer circuit as set forth in claim 8 includingband-limiting means for placing an upper frequency limit on the band offrequencies within which said capacitance simulating circuit cancelssaid stray capacitance.

12. A compensated transfonner circuit as set forth in claim 8 whereinone of the feedback impedances comprises a reactance and wherein theremaining feedback impedances comprise resistors.

13. A compensated transformer circuit as set forth in claim 12 includingband-limiting means for cancelling the effect of said reactance forfrequencies beyond the band of frequencies within which the cancellationof stray capacitance is desirable.

14. A compensated transformer circuit as set forth in claim 8 whereinsaid negative feedback impedance comprises an inductor and wherein saidpositive and input feedback impedances comprise resistors.

15. A compensated transformer circuit as set forth in claim 8 whereinsaid input feedback impedance comprises a capacitor and wherein saidpositive and negative feedback impedances comprise resistors.

16. A compensated transformer circuit as set forth in claim 8 whereinsaid positive feedback impedance comprises a capacitor and wherein saidnegative and input feedback impedances comprise resistors.

17. A compensated transformer circuit as set forth in claim 8 whereinsaid positive feedback impedance comprises an inductor and wherein saidinput and negative feedback impedances comprise resistors.

18. A compensated transformer circuit as set forth in claim 8 whereinsaid negative feedback impedance comprises a capacitor and wherein saidpositive and input feedback impedances comprise resistors.

1. A compensated transformer circuit comprising, in combination, atransformer including a primary and a secondary winding, capacitancesimulating means for establishing voltages and currents which affect thecircuitry to which said voltages and currents are applied as if anegative capacitance were connected to said circuitry, said negativecapacitance having a magnitude substantially proportional to the straycapacitance of said transformer, and coupling means for coupling saidcapacitance simulating means to said transformer to substantially cancelthe effect which said stray capacitance has on the voltages across andcurrents through said primary and secondary wiNdings.
 2. A compensatedtransformer circuit as set forth in claim 1 in which said coupling meanscomprises a tertiary winding in said transformer.
 3. A compensatedtransformer circuit as set forth in claim 1 in which said coupling meansincludes means for connecting said capacitance simulating means acrosssaid primary winding.
 4. A compensated transformer circuit as set forthin claim 1 in which said coupling means includes means for connectingsaid capacitance simulating means across said secondary winding.
 5. Acompensated transformer circuit as set forth in claim 1 includingband-limiting means for placing an upper frequency limit on the band offrequencies within which said capacitance simulating means cancels saidstray capacitance.
 6. A compensated transformer circuit including, incombination, a transformer having a primary and a secondary winding, acapacitance simulating circuit comprising a plurality of terminals, anamplifier, a plurality of feedback elements and means for connectingsaid feedback elements to said amplifier to establish between saidterminals an effective negative capacitance having a magnitudesubstantially proportional to the stray capacitance of said transformer,said compensated transformer circuit further including coupling meansfor coupling said terminals to said transformer to substantially cancelthe effect which said stray capacitance has on the transformation ofvoltage and current in said transformer.
 7. A compensated transformercircuit as set forth in claim 6 including band-limiting means forplacing an upper frequency limit on the band of frequencies within whichsaid capacitance simulating circuit cancels said stray capacitance.
 8. Acompensated transformer circuit as set forth in claim 6 in which saidamplifier is an operational amplifier of the type which includes aninverting input, a non-inverting input and an output and in which saidplurality of feedback elements includes a positive feedback impedance, anegative feedback impedance and an input feedback impedance.
 9. Acompensated transformer circuit as set forth in claim 8 wherein saidinput feedback impedance is connected to said inverting input.
 10. Acompensated transformer circuit as set forth in claim 8 wherein saidinput feedback impedance is connected to said noninverting input.
 11. Acompensated transformer circuit as set forth in claim 8 includingband-limiting means for placing an upper frequency limit on the band offrequencies within which said capacitance simulating circuit cancelssaid stray capacitance.
 12. A compensated transformer circuit as setforth in claim 8 wherein one of the feedback impedances comprises areactance and wherein the remaining feedback impedances compriseresistors.
 13. A compensated transformer circuit as set forth in claim12 including band-limiting means for cancelling the effect of saidreactance for frequencies beyond the band of frequencies within whichthe cancellation of stray capacitance is desirable.
 14. A compensatedtransformer circuit as set forth in claim 8 wherein said negativefeedback impedance comprises an inductor and wherein said positive andinput feedback impedances comprise resistors.
 15. A compensatedtransformer circuit as set forth in claim 8 wherein said input feedbackimpedance comprises a capacitor and wherein said positive and negativefeedback impedances comprise resistors.
 16. A compensated transformercircuit as set forth in claim 8 wherein said positive feedback impedancecomprises a capacitor and wherein said negative and input feedbackimpedances comprise resistors.
 17. A compensated transformer circuit asset forth in claim 8 wherein said positive feedback impedance comprisesan inductor and wherein said input and negative feedback impedancescomprise resistors.
 18. A compensated transformer circuit as set forthin claim 8 wherein said negative feedback impedance comprises acapacitor and wherein said positive and input feedback impedancescomprise resistoRs.